I think it got changed into the mature category because of the most recent chapter content. It's still available via the app but you'll need to make sure you've got your mature content setting turned on.

You'll probably also need to logout, close the app completely then reopen it and login. While the series will probably still not show up right away in "Popular," it should show up in the "Recently Launched" Sort (and in whatever genre it's categorized as). It will also show up in Popular once you change the sort to anything else then back to Popular--because that forces the view to refresh.